Current-source power converting apparatus with self-extinction devices

ABSTRACT

A current-source power converting apparatus having a three phase AC-DC converter composed of self-extinction devices for converting an AC power furnished from an three phase AC power source into a DC power and a DC-AC inverter connected with the AC-DC converter through a DC reactor for re-converting the DC power into a three phase AC power to supply the re-converted power for a load. When the failure of the AC power source is detect according to one embodiment, the AC power source is detached from the AC-DC converter and a battery is connected between arbitrary two phases at the input end of the AC-DC converter. After that, the DC power of the battery is supplied for the DC-AC inverter intermittently by switching the corresponding self-extinction devices of the AC-DC converter and controlled by varying the duty ratio of the switching operation.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a current-source power convertingapparatus with self-extinction devices, and more particularly to thepower converting apparatus which includes an AC-DC converter forconverting an alternating current power furnished from an AC powersource into a direct current power and a DC-AC inverter connected withthe AC-DC converter through a direct current reactor for re-convertingthe direct current power into an alternating current power to supply thereconverted power for a load, and which is suitable for continuingoperation of the DC-AC inverter even at the time of failure of the ACpower source.

2. Description of the Related Art

There is a demand that, even if an AC power source fails, a powerconverting apparatus fed from the AC power source must continue tooperate for a while. For example, a driving motor for an elevator isrequired to continue to operate until an elevator cage running at thattime reaches the most neighboring floor safely. The power convertingapparatus supplying such a driving motor with the electric power has tocontinue the feeding of the necessary power, the performance thereofbeing more or less derated.

Usually, two types of the power converting apparatus are known; one typeis a so called voltage-source type and the other a current-source type.The former has been used rather more frequently from the reason asfollows.

In the voltage-source power converting apparatus, an AC-DC converterincluded in such apparatus has not been required to be capable ofcontrolling its output DC voltage. The AC-DC converter was sufficientonly to output the DC power of the constant voltage, because the controlof the voltage applied to a load can be easily realized by a DC-ACinverter connected to the AC-DC converter. Accordingly, at the time offailure of an AC power source, the AC-DC converter is replaced by abattery which can supply the DC-AC inverter with the DC power of theconstant voltage, and the voltage of the AC power supplied for the loadis controlled by the usual control method of the DC-AC inverter.

However, when the voltage-source power converting apparatus conducts aregenerative operation, it becomes necessary to provide anotherconverter exclusively used for the regenerative operation. On the otherhand, in the current-source one, the power converting apparatus canachieve the regenerative operation by only the gate control of oneconverter without any further converter. Therefore, the current-sourcepower converting apparatus is used, when the regenerative operation isrequired.

Contrary to the case of the voltage-source type, however, a DC-ACinverter used in a current-source power converting apparatus is verydifficult to control the voltage of its output AC power. Such controlhas been scarcely feasible in a practical use. Therefore, an AC-DCconverter connected with the DC-AC inverter through a DC reactor has tofill the role of voltage control of the AC power supplied for a load asthe final output of the current-source power converting apparatus. Thatis to say, the voltage of the DC power furnished for the DC-AC inverterhas to be controlled by the AC-DC converter. Accordingly, the DC-ACinverter can not continue to operate by merely substituting a batteryfor the AC-DC converter at the time of failure of an AC power source.Such substitution of the battery for the converter made possible thecontinuous operation of the inverter at the time of failure of the ACpower source in a case of the foregoing voltage-source power convertingapparatus.

Now, in a conventional example, a current-source power convertingapparatus employing self-extinction devices utilizes the DC shortcircuiting mode of operation for a usual control of the DC voltage bymeans of the AC to DC conversion. In order to make such a powerconverting apparatus operate continuously at the time of failure of anAC power source, a battery is connected to the apparatus and the controlof DC voltage must be performed by the DC to DC transformation. Since,however, the DC to DC transformation necessiates the provision of afreewheel diode, the arrangement of a main circuit of the usualconverter is not suited for control of the DC voltage. Further, in casethe freewheel diode is connected to the output end of the AC-DCconverter, there arises a defect that the regeneration becomesimpossible in the usual operation.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a current-source powerconverting apparatus which comprises an AC-DC converter connected to anAC power source and a DC-AC inverter supplied with the DC power from theAC-DC converter through a DC reactor, at least the AC-DC converter beingcomposed of self-extinction devices, and which is capable of making theDC-AC inverter operate continuously by supplying the DC power from abattery at the time of failure of the AC power source.

According to a feature of the present invention, there is provided inthe current-source power converting apparatus as described above a DCpower supplying means which is capable of supplying the DC power of theintermittent voltage for a load during the failure of the AC powersource, wherein the DC power supplied for the load is controlled byvaring the degree of the intermittence. Further the DC power supplyingmeans includes a battery for supplying the DC power which is connectedon the side of the AC-DC converter with respect to the DC reactorarranged between the AC-DC converter and the DC-AC inverter.

In one of the embodiments according to the present invention, the DCpower supplying means supplies the DC power of the battery for theinverter intermittently by using the self-extinction devices of theAC-DC converter.

According to another embodiment, the DC power supplying means includes aparticular switching means for supplying the DC power of the battery forthe inverter intermittently.

According to still another embodiment, the DC power supplied for theDC-AC inverter from the battery is controlled intermittently by theDC-AC inverter itself.

Other objects and features of the present invention will become apparentupon reading the specification and inspection of the drawings and willbe particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a circuit of a current-sourcepower converting apparatus according to an embodiment of the presentinvention;

FIG. 2 shows the waveforms of signals and voltage/current which areapplied to or derived from various parts of the circuit shown in FIG. 1,and the figure includes those before and after occurence of the failureof an AC power;

FIGS. 3a and 3b are diagrams for the purpose of explanation of theoperation of the circuit shown in FIG. 1;

FIG. 4 is a schematic diagram showing a circuit of a current-sourcepower converting apparatus according to another embodiment of thepresent invention;

FIG. 5 shows the waveforms of signals and voltage/current which areapplied to or derived from various parts of the circuit shown in FIG. 4,and the figure includes those before and after occurence of the failureof an AC power;

FIGS. 6a and 6b are diagrams for explaining the operation of the circuitshown in FIG. 4;

FIG. 7 is a schematic,. diagram showing a circuit of a current-sourcepower converting apparatus according to still another embodiment of thepresent invention;

FIG. 8 shows the waveforms of signals and voltage/current which areapplied to or derived from various parts of the circuit shown in FIG. 7,and the figure includes those before and after occurence of the failureof an AC power;

FIG. 9, shows the waveforms of signals and current which are applied toor derived from various parts of a DC-AC inverter used in the circuitshown in FIG. 7 during the failure of the AC power;

FIG. 10 is an expanded diagram of a part of the drawing of FIG. 9; and

FIGS. 11a, 11b and 11c are diagrams for explaining the operation of theDC-AC inverter used in the circuit shown in FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be explained in detail withreference to the accompanying drawings, wherein like parts in each ofthe several figures are identified by the same reference numerals andcharacters.

Referring at first to FIG. 1, a reference numeral 1 denotes a threephase AC power source, which supplies a three phase AC power for anAC-DC converter 3 through a three phase contactors 5. At an input end ofthe converter 3, three capacitors are provided, which are formed in aknown star-connection and denoted as a whole by a reference numeral 7.

The AC-DC converter 3 is formed by a three phase bridge circuit, eacharm of which is composed of a so called self-extinction device, such asa gate turn-off thyristor or a transistor. The self-extinction devicesincluded in the respective arms are denoted by reference numerals 31,32, 33, 34, 35 and 36. Since such an arrangement of the converter iswell known, the further description thereabout is omitted here. Abattery is provided between arbitrary two among three phases U, V and Wof the AC power in order that it supplies a DC power for the converter 3when the AC power source 1 fails. In this embodiment, the battery 11 isconnected between the phases V and W through a contactor 13.

The DC power as an output of the converter 3, voltage and current ofwhich are represented by reference characters V_(d) and I_(d),respectively, is supplied to a DC-AC inverter 9 through a DC reactor 15which smooths the DC current I_(d) flowing therethrough. In thisembodiment, the DC-AC inverter 9 is also formed by a three phase bridgecircuit which has six arms each including a self-extinction device. Thearrangement of the inverter of this kind is also known. The inverter 9inverts the DC power supplied from the converter 3 into an AC power,which is furnished to a load, i.e., a three phase induction motor 17 ina case of this embodiment. Further, capacitors, denoted as a whole by areference numeral 19, are connected at an output end of the inverter 9.

There is further provided a control unit 21 consisting of a powerfailure detector 23 and a control circuit 25. The power failure detector23 has a sensing part (not shown) equipped at an appropriate portion ofthe main circuit where the failure of the AC power can be watched. Whenthe power failure occurs, the detector 23 produces output signals. Oneof the output signals is applied to the contactor 5, so that thecontactor 5 is opened to detach the AC power source 1 from the converter3. Another output signal is given to the contactor 13 to close it.Thereby the battery 11 is connected between the phases V and W. The lastoutput signal is led to the control circuit 25 as a signal indicatingoccurence of the failure of the AC power.

The control circuit 25 consists mainly of gate control means for theconverter 3 and the inverter 9. During the normal condition of the ACpower source 1, the control circuit 25 operates in almost the samemanner as a known gate control means for the converter or inverter ofthis kind. The operation thereof at the time of the power failure willbe explained in detail later.

Next, the description is made of the operation of this embodiment,referring to FIG. 2 and FIGS. 3a and 3b.

In FIG. 2, it is assumed that the failure of the AC power occurs at atime point t₀, and therefore, through all figures of FIGS. 2 (a) to (g),the waveforms illustrated in the duration before the time point t₀ arethose during the normal operation and the waveforms in the durationafter the time point t₀ are those under the condition of the powerfailure. Further, in these figures, FIG. 2 (a) shows the waveform ofvoltage of the AC power source 1, in which the broken lines V_(U), V_(V)and V_(W) represent phase voltages of the phases U, V and W,respectively, and the solid line V_(VW) the line voltage with respect tothe phases V and W between which the battery 11 is connected.

Gate signals of the converter 3 are as shown in FIG. 2 (b). The gatesignals P₃₁, P₃₂, P₃₃, P₃₄, P₃₅ and P₃₆ are applied to gates of theself-extinction devices 31, 32, 33, 34, 35 and 36 of the converter 3,respectively. As is understood from the application manner of these gatesignals, a so called PWM (Pulse Width Modulation) control method isapplied to the converter 3 of this embodiment in order to adjust itsoutput DC voltage during the normal operation.

The power failure detector 23 produces an output as shown in FIG. 2 (c)at the time point t₀ when it detects the power failure, and thecontactors 5 and 13 operate as shown in (d) and (e) of FIG. 2,respectively, upon occurence of the output of the detector 23. VoltageV_(d) and current I_(d) of the DC power as an output of the converter 3become as shown in (f) and (g) in the same figure.

Now, when the power failure is detected by the detector 23 at the timepoint t₀, the contactor 5 which was of ON state till then changes to OFFstate and, on the contrary, the contactor 13 becomes ON state. Namely,the AC power source 1 is detached from the converter 3 and the battery11 is connected between the phases V and W of the converter 3. As aresult, the line voltage V_(VW) between the phases V and W becomes equalto voltage E of the battery 11 (cf. FIG. 2 (a)).

After the time point t₀, the self-extinction device 36 is kept at ONstate by applying the continuing gate signal thereto (cf. P₃₆ in FIG. 2(b)), and the self-extinction devices 32 and 33 repeat ON and OFF statesalternately (cf. P₃₂ and P₃₃ in the same). Here assuming that the gatesignal P32 becomes a high level at a time point t₁ and the gate signalP₃₃ at a time point t₂, i.e., that the self-extinction device 32 becomesconductive at t₁ and the self-extinction device 33 turns on at t₂. Inthis case, the states of the circuit of the converter 3 for thedurations of t₀ ≦t<t₁ and t₁ ≦t<t₂ are as shown in FIGS. 3a and 3b,respectively.

As is apparent from FIG. 3a, the self-extinction devices 33 and 36 areboth in ON state, so that a DC circuit side falls into the short circuitstate and the voltage V_(d) becomes zero. At this time, the seriesconnection of the self-extinction devices 33 and 36 functions as afreewheel diode, when being viewed from a side of the load. Therefore,this state of the circuit is called a freewheel-state. FIG. 3b shows thecircuit state for the duration of t₁ ≦t<t₂. In this duration, theself-extinction device 36 is continuously conductive. Theself-extinction device 33 changes to OFF state and the self-extinctiondevice 32 becomes ON state. Accordingly, the battery 11 is connected tothe DC circuit, so that the voltage V_(d) becomes equal to the voltage Eof the battery 11.

As is understood from FIG. 2, especially from P₃₂ and P₃₃ in the figure(b) thereof, the operation as described above is repeated over theduration of the power failure. As a result, the DC power with thevoltage V_(d) and the current I_(d) as shown in (f) and (g) of FIG. 2 isobtained from the converter 3, and the DC power thus obtained issupplied for the inverter 9 through the DC reactor 15. Here, if the ONterm of the self-extinction device 32, and accordingly the OFF term ofthe self-extinction device 33, is varied, the pulse width of the DCvoltage V_(d) changes so that the average value of the DC voltage V_(d)changes. In this manner, the voltage of the DC power supplied for theinverter 9 can be controlled even at the time of failure of the ACpower.

According to this embodiment, the main circuit of the current-sourcepower converting apparatus is additionally provided by only a DC powersupplying means as measures against the failure of the AC power, whichis simply constructed by the battery 11 and the contactor 13. The maincircuit usually has the contactor 5 as a main switch and therefore thesignal from the power failure detector 23 can be applied to an actuatorfor the main switch. Accordingly, the main circuit is considerablysimple in its structure, since the voltage control is performed by thegate control of the self-extinction devices of the converter 3. Acircuit for such gate control as renders the self-extinction devices 32and 33 conductive alternately and controls the duty ratio of the pulsingDC voltage V_(d) can be easily achieved with a usually known electroniccircuit techniquue.

FIG. 4 shows a current-source power converting apparatus according toanother embodiment of the present invention. In this figure, the detailsof a converter 3 and an inverter 9 are omitted, since they are entirelythe same as those in FIG. 1.

Referring to this figure, a DC power supplying means 4 as measuresagainst the failure of the AC power is provided at the output end of theconverter 3, i.e. in the DC circuit of the power converting apparatus.The DC power supplying means 4 comprises a series connection of abattery 41 and a self-extinction device 42 and another self-extinctiondevice 43 connected in parallel with the series connection. Theself-extinction device 42 has a diode 44 connected in reverse paralleltherewith. Further, this arrangement of circuit is connected acrossoutput terminals of the converter 3 through contactors 45 and 46.

A power failure detector 23 in this embodiment produces four outputsignals, when it detects the power failure. The first output signal issent to a contactor 5 to make it open and detach an AC power source 1from the converter 3. The second output signal is given to thecontactors 45 and 46 to close them. The third one is a gate signal forthe self-extinction devices 42 and 43, which renders these devices 42,43 conductive alternately, as described more in detail later. The lastoutput signal of the detector 23 is led to a control circuit 25 as asignal indicating the occurence of the power failure. The first, secondand third among the four output signals of the detector 23 are almostthe same in their function as the signals produced by the power failuredetector in FIG. 1.

Referring now to FIG. 5 and FIGS. 6a and 6b, the description will bemade of the operation of this embodiment, hereinafter.

Similarly to a case of FIG. 2, it is assumed in FIG. 5 that the failureof the AC power occurs at a time point t₀, and therefore, through allfigures of FIGS. 5(a) to (i), the waveforms illustrated in the durationbefore the time point t₀ are those during the normal operation and thewaveforms in the duration after the time point t₀ are those under thecondition of the power failure. Further, FIGS. 5(a) to (e), (h) and (i)are the same as the corresponding figures of FIG. 2. As is apparent fromthese figures, the operation during the normal conditon is the same asthat of the first embodiment shown in FIG. 1. Therefore, the explanationthereabout is omitted.

When the power failure occurs at the time point t₀, the detector 23produces the output signal (cf. FIG. 5(c)). In response to the outputsignal, the contactor 5 becomes OFF state and the contactors 45 and 46become ON state (cf. FIGS. 5(d) and (e)). Namely, the AC power source 1is detached from the converter 3 and the DC power supplying means 4 isconnected to the inverter 9 through a DC reactor 15. Further, as shownin FIG. 5(b), the gate signals P₃₁ P₃₂, P₃₃, P₃₄, P₃₅ and P₃₆ to theconverter 3 are all suppressed. After the time point t₀, theself-extinction devices 42 and 43 are given their gate signals P₄₂ andP₄₃ (cf. FIGS. 5(f) and (g)). Here assuming that the gate signal P₄₂becomes a high level at a time point t_(l) and the gate signal P₄₃ at atime point t₂, that is to say, that the self-extinction device 42becomes conductive at t₁ and the self-extinction device 43 at t₂. Inthis case, the state of the circuit of the converter 3 for the durationsof t₀ ≦t<t₁ and t₁ ≦t<t₂ are as shown in FIGS. 6a and 6b, respectively.

In the duration of t₀≦t<t₁, (cf. FIG. 6b), the self-extinction device 43is in ON state, therefore a DC circuit side falls into the short circuitstate and the voltage V_(d) becomes zero. At this time, theself-extinction device 43 functions as a freewheel diode when beingviewed from a side of the load. This state is the same as that shown inFIG. 3a, and therefore, this is also called a freewheel-state. FIG. 6billustrates the circuit state for the duration of t₁≦t<t₂, in which theself-extinction device 42 is in ON state and the battery 41 is connectedto the inverter 9. In this duration, the DC voltage V_(d) becomes equalto the voltage E of the battery 41.

As is seen from FIGS. 5(f) and (g) showing the gate signals P₄₂ and P₄₃applied to the self-extinction devices 42 and 43, the operation asmentioned above is repeated over the duration of the power failure. As aresult, the DC power of the voltage V_(d) and the current I_(d) as shownin FIGS. 5(h) and (i) are obtained from the DC power supplying means 4,and the DC power thus obtained is supplied for the inverter 9 throughthe DC reactor 15. Similarly to the case in FIG. 2, the average value ofthe output voltage of the DC power supplying means 4 can be adjusted bycontrolling the duty ratio of the pulsing DC voltage V_(d).

According to the second embodiment, the voltage of the DC power suppliedfor the inverter 9 can be controlled, not only when the AC power source1 fails, but also at the time of the trouble of the converter 3.

FIG. 7 is a schematic diagram showing a current-source power convertingapparatus in accordance with still another embodiment of the presentinvention, in which, similarly to FIG. 4, the details of a converter 3is omitted. However, an inverter 9 is illustrated in detail although itis the same as that in FIG. 1, because the following description of thisembodiment is concerned with the control manner of the inverter 9.Therefore, the detailed illustration of the converter 9 in this figureis only for the purpose of convenience of understanding of thisembodiment.

In the embodiment of FIG. 7, there is provided a DC power supplyingmeans 6 of the simpler structure, compared with the DC power supplyingmeans 4 in FIG. 4. Namely, the DC power supplying means 6 in thisembodiment comprises a battery 61 and contactors 62, 63 connecting thebattery 61 with a DC circuit of the power converting apparatus. Thecontactors 62, 63 are closed in response to a signal from a powerfailure detector 23, so that the DC power is supplied to an inverter 9from the battery 61 through a DC reactor 15 at the time of failure of anAC power source 1.

Referring to FIG. 8, the explanation is done of the operation of thisembodiment. As is understood from FIG. 8(b), the converter 3 is operatedin the same manner as those in FIGS. 1 and 4, i.e. in the PWM mode,during the normal condition of the AC power source 1. However, theconverter 3 in this embodiment is so controlled that its output DC poweris maintained constant. The DC voltage V_(d) and the DC current I_(d) ofthe output DC power of the converter 3 are as shown in FIGS. 8(f) and(g), and the constant DC power is supplied for the inverter 9 throughthe DC reactor 15.

On the other hand, the inverter 9 is given gate signals P₉₁, P₉₂, P₉₃,P₉₄, P₉₅ and P₉₆ as shown in FIG. 8(h) and operated in the PWM mode.Consequently, the inverter 9 produces the output currents I_(U), I_(V),I_(W) as shown by rectangular waveforms of FIG. 8(i) and the currentsflowing through a load 17 become as I_(UL), I_(VL) and I_(WL) shown bybroken sinusoids of the same figure. Different from the inverters in twoembodiments already described, the inverter 9 in this embodiment is ableto control its output AC power by means of the PWM control operation.

Now, when the failure of the AC power source 1 is detected at the timepoint t₀ (cf. FIG. 8(a)), the detector 23 produces an output signal (cf.FIG. 8(c)). In reply to this output signal, the contactor 5 is openedand the contactors 62, 63 are closed so that the AC power source 1 isreleased and the battery 61 is connected to the DC circuit. Accordingly,after that, the DC voltage V_(d) is kept at the voltage E of the battery61 (cf. FIG. 8(f)). Further, after the time point t₀ the gate signalsP₃₁, P₃₂, P₃₃, P₃₄, P₃₅ and P₃₆ of the converter 3 are all suppressed,similarly to the case of FIG. 5. However, the gate signals P₉₁, P₉₂,P₉₃, P₉₄, P₉₅ and P₉₆ continue to be applied to the correspondingself-extinction devices 91, 92, 93, 94, 95 and 96 of the inverter 9 (cf.FIG. 8(h)). Therefore, the output AC power of the inverter 9 iscontinuously controlled (cf. FIG. 8(i)).

Referring next to FIG. 9, the AC output power control by the inverter 9after the time point t₀, i.e. during the failure of the AC power, isexplained hereinafter.

In this figure, distribution signals R_(U), R_(Z), R_(V), R_(X), R_(W)and R_(Y) (cf. FIG. 9(a)) are signals each of which has a pulse width Tcorresponding to an operational period 60° of the inverter 9 and isshifted by 60° in phase from one another. These distribution signals areobtained by dividing one cycle of the voltage of the AC power source 1into six equal sections I to VI. A signal Q (cf. FIG. 9(c)) is atriangular wave signal which has a peak or a maximum value I_(RMAX) atthe time point of the leading edge of pulses of a reference pattern Pfor the PWM control (cf. FIG. 9(b)). The periods of individualtriangular waves of the signal Q are determined by the width of thepulses of the reference pattern P and hence not uniform. A short-circuitpulse train S is made by comparing the triangular wave signal Q with aninstruction I_(R) ^(*) of the AC output current (cf. FIGS. 9(c) and(d)). Next, the reference pattern P and the short-circuit pulse train Sare inverted into signals P and S, respectively. By taking the logicalproduct between the signals P and S and between the signals P and S,signals P_(F) and P_(R) are obtained (cf. FIGS. 9(e), and (f)).

Further, signals P'_(F), P'_(R) and S' are obtained by the logicalproduct of the signals P_(F) and R_(Y), of the signals P_(R) and R_(Z),and of the signals S and R_(X) (cf. FIGS. 9(g), (h) and (i)), and thelogical summation of the thus obtained signals P'_(F), P'_(R), S' andthe signal R_(U) is conducted to get the gate signal P₉₁ to theself-extinction device 91 (cf. FIG. 9(j)). In the similar way, the gatesignals P₉₂, P₉₃, P₉₄, P₉₅ and P₉₆ of the self-extinction devices 92,93, 94, 95 and 96 of the inverter 9 can be made. The output current ofthe inverter 9 controlled by the gate signals P₉₁ to P₉₆ becomes thepulse-width-modulated rectangular current as shown by I_(U), I_(V),I_(W) in FIG. 9(k). The currents flowing through the load are as shownby broken sinusoids I_(UL), I_(VL), I_(WL) in the same figure.

In order to facilitate a good understanding, the output control in theinverter 9 is explained more in detail, referring to FIG. 10 which,taking the section I shown in FIG. 9 as an example, illustrates theoperation in the expanded form and to FIGS. lla to llc which indicatethe circuit states in the duration from a time point t₀ to a time pointt₃ shown in FIG. 10.

Referring at first to FIG. 10, the short-circuit pulse train S obtained,in this section I, by comparing the triangular wave signal Q with thecurrent instruction I_(R) ^(*) corresponds to the gate signal P₉₂ givento the self-extinction device 92 (cf. FIGS. 10(b) and (e)). On the otherhand, in the section I, the self-extinction device 95 is continuouslygiven the gate signal P₉₅, as shown in FIG. 10(h). Therefore, the DCcircuit of the power converting apparatus is short-circuited, as shownin FIG. llb which shows the circuit state of the inverter 9 in theduration from the time point t₁ to the time point t2. Accordingly, thecurrents IU, IV, IW in each phase U, V, W of the load become zero inthis duration (cf. FIGS. 10(i), (j) and (k)). Before this duration, i.e.in the durarion from the time point t₀ to the time point t_(l), theself-extinction devices 93 and 95 are made conductive (FIGS. 10(g) and(h)), so that the current flows through the load/in the phases V and W(cf. FIGS. 10(j) and (k) and FIG. ll(a). In the duration from the timepoint t₂ to the time point t3, the self-extinction devices 91 and 95become ON state (cf. FIGS. 10(c) and (g)), so that the current flowsthrough the load in the phases U and V (cf. FIGS. 10(i) and (j), andFIG. ll(c).

As ;is apparent from FIG. 10, the inverter 9 is able to control thecurrent flowing throught the load by successively repeating the threecircuit states as shown in FIGS. lla, llb and llc.

Now, assuming here that the conductive and non-conductive durations ofthe current in the respective phases are denoted by d₀ to d₃ and d_(S1)to d_(S3) as shown in FIGS. 10(i) to (k). Then, if the currentinstruction I_(R) ^(*) is varied from zero to the maximum valueI_(RMAX), the short circuit durations d_(S1), d_(S2), d_(S3) in thecurrent I_(U) of the phase U changes from d₁, d₂ and d₃ to zero,respectively, holding the following relation: ##EQU1## The same relationis applicable to the current I_(W) of the phase W. With respect to thecurrent IV of the phase V, the following relation exists between d₁ tod₃ and d_(Sl) to d_(S3) : ##EQU2##

In these circumstances, the effective value I_(RMS) of the currentI_(U), I_(V), I_(W) becomes as follows: ##EQU3## When I_(R) ^(*) =0, theDC circuit of the power converting apparatus is in the short circuitstate and therefore I_(RMS) becomes equal to zero. Contrary, when I_(R)^(*) =I_(RMAX), I_(RMS) becomes as follows: ##EQU4##

Namely, as is apparent from the equation (3) above, the effective valueIRMS of the current I_(U), I_(V), I_(W) changes in proportion to asquare root of the current instruction I_(R) ^(*), since I_(d), I_(RMAX)and d₀ to d₃ are all constant.

As described above, the continuous operation of the power convertingapparatus during the failure of the AC power is possible by onlyconnecting the battery 61 in case the inverter 9 itself is able tocontrol the output power.

Further, in this embodiment, there is an additional feature as follows.Namely, in this embodiment, the short circuit state of the DC circuitincludes the battery 61 within the circuit (cf. FIG. 7 and FIG. 11 b).Therefore, energy is stored in the DC reactor 15 by the current I_(d)flowing therethrough during the short circuit state. When the shortcircuit state is broken, that is to say, the circuit state changes fromthe state shown in FIG. 11b to that shown in FIG. 11c, for example, theenergy stored in the DC reactor 15 is released toward the load 17. Atthis time, voltage (L dI_(d) dt) is produced across the DC reactor 15,wherein L represents an inductance value of the DC reactor 15.Therefore, the condensor 19 connected to the output end of the inverter9 can be charged by the sum of the voltage E of the battery 61 and thevoltage (L dI_(d) / dt) produced by the DC reactor 15. Accordingly, theload 17 can be applied by the voltage higher than the voltage E of thebattery 61.

Although we have herein shown and described only limited number of formsof a current-source power converting apparatus embodying the presentinvention, it is to be understood that various changes and modificationsmay be made therein within the scope of the appended claims withoutdeparting from the spirit and scope of the present invention.

We claim:
 1. A current-source power converting apparatuscomprising:converter means including a plurality of self-extinctiondevices, said converter means being supplied by an AC power source forconverting AC power into DC power; inverter means including a pluralityof self-extinction devices for inverting the DC power converted by saidconverter means into AC power and for supplying the inverted AC power toan AC load; DC reactor means connected between said converter means andsaid inverter means; capactor means connected at the output of saidinverter means; and DC power supply means including a battery forsupplying DC power and means for intermittently communicating the DCpower between said battery and said inverter means when the AC powersource fails.
 2. A current-source power converting apparatus accordingto claim 1, wherein said DC power supply means includes said batteryconnected at the AC side of said converter means and said means forintermittently communicating the DC power controlling saidself-extinction devices of said converter means for intermittentlycommunicating the DC power through said DC reactor means when the ACpower source fails.
 3. A current-source power converting apparatusaccording to claim 1, wherein said DC power supply means includes acircuit comprising said battery and at least one other self-extinctiondevice connected across output terminals of said converter means, saidmeans for intermittently communicating the DC power controlling the atleast other self-extinction device of said circuit for intermittentlycommunicating the DC power through said DC reactor means when the ACpower source fails.
 4. A current-source power converting apparatusaccording to claim 1, wherein said DC power supply means includes saidbattery connected across output terminals of said converter means andsaid means for intermittently communicating the DC power controllingsaid inverter means for intermittently communicating the DC power whenthe AC power source fails.